This invention relates to a system for recovering heat from combustion gases or combustible gases produced by partial burning of combustibles. In particular, the invention relates to a heat recovery system that can be applied to the treatment of municipal solid wastes (so-called municipal wastes" or MW) or waste plastics.
The reduction of dioxins and the rendering of soot and dust innocuous are two essential requirements that must be met by recent waste incineration systems. In addition, it has been proposed that new thermal recycling systems be established that can treat wastes not only as materials to be disposed of but also as alternative energy sources.
Advanced power generation systems using municipal wastes have been developed with a view to generating electricity at a higher rate of efficiency than conventional systems in the process of burning solid wastes. According to a modified version of this system that utilizes reburning and superheating, the steam produced in a waste heat boiler is superheated to a higher temperature with a clean hot combustion gas produced by reburning combustion gas from a combustion furnace using high-grade fuel of different origin, for example, kerosine or natural gas. Such an independent superheater is used for the purpose of enhancing the efficiency of power generation with steam turbines. The advanced system of power generation from municipal waste utilizing such superheating method is under active development as being suitable for incineration facilities of a comparatively small scale.
Gases produced in the combustion of municipal wastes generally contain HCl which is generated by the combustion of polyvinyl chloride, and if the surface temperature of heat transfer pipes for heat recovery exceeds about 400.degree. C., corrosion of these pipes due to HCl becomes pronounced. To avoid this problem, the temperature of superheated steam must be held lower than 400.degree. C., but as a result increased efficiency of power generation with steam turbines cannot be achieved.
However, a recent study has revealed that the main cause of corrosion of heat transfer pipes is in fact the deposit of molten salts on the pipes. Municipal wastes have high concentrations of salts such as NaCl (m.p. 800.degree. C.) and KCl (m.p. 776.degree. C.) and, as the combustion proceeds, these salts form a fume and are deposited on the heat transfer pipes, the temperature of which is low. Since this deposit accelerates the corrosion of the heat transfer pipes, the maximum temperature of the superheated steam that can be used in the existing power generation systems using municipal wastes has been about 300.degree. C., which will ensure that the surface temperature of heat transfer pipes can be held below about 320.degree. C.
Table 1 compares the features of various thermal recycling systems. Obviously, for successful high-efficiency power generation and RDF (refuse-derived fuel) power generation, the use of higher-grade materials as heat transfer pipes is not sufficient and conditions preventing the above discussed corrosion problem must first be realized.
TABLE 1 __________________________________________________________________________ Power Generating Method Details Features Comments __________________________________________________________________________ Conventional power The heat of combustion is Steam pressure is low because Once a superheated steam generation recovered by a waste heat boiler to the superheated steam temperature of 400.degree. C. is generate electricity using back temperature has conventionally assured, high steam pressure steam turbines. been set to a low level. As pressures also will be result, the power generating attained. efficiency is also low. In recent years, heated steam at a temperature of 400.degree. C. has been attempted. Highly efficient generation New material developments have No additional load on the The development of by new material led to materials for incineration environment, assist fuel is materials resistant to molten development furnaces and superheaters that are required. salt corrosion encounters resistant to corrosive components both technical and economic such as hydrochloric acid which are difficulties. It is therefore generated in the combustion of more important to create refuse/wastes. This has led to conditions that will avoid improvements in steam conditions corrosion. and enhancement of power generating efficiency. RDF Power Generation The addition of lime and the like to As it is difficult to generate Though hydrochloric acid the waste material to produce a electricity at a high efficiency formation is decreased, the solid fuel not only has the in a small-scale plant, only measures against molten salt advantage of helping to prevent solid refuse material is corrosion are practically at putrefaction but also helps to create produced. The RDF is the same level as before. It more favorable steam conditions therefore collected for is therefore necessary to with a view to achieve a higher generating electricity at high create conditions that will level of power generating efficiency efficiency in a large-scale obviate corrosion as by dechlorination and plant. described above. desulfurization. Advanced Refuse Power Combined cycle power generation The most effective practical The use of large amounts of Generation with gas turbine. Power is is to introduce such a system high quality fuels and the generated with a gas turbine, and large-scale incineration economic feasibility of the waste heat from the gas turbine is systems. This process requires process are problems. The utilized to superheat the steam from gas turbine fuel such as natural key is whether the unit price the refuse waste heat boiler. By gas. of produced electricity is this means, the efficiency of power increased. generation is enhanced. Reburning by use of an This is included in an Advanced This method offers a high fuel The use of large amounts of Additional Fuel Refuse Power Generating system. utilization efficiency and is high quality fuels is The steam from the waste heat effective in small-scale expensive. The key is to boiler is superheated by using incineration plants. ensure that the price at additional separate fuel in order to which the power sold is enhance the power generating greater than the fuel costs. efficiency of the steam turbine. __________________________________________________________________________
The advanced systems of power generation from MW involve huge construction and fuel costs and hence require thorough preliminary evaluation of process economy. Deregulation of electric utilities is a pressing need in Japan but, on the other hand, the selling price of surplus electricity is regulated to be low (particularly at night). Under these circumstances, a dilemma exists in that high-efficiency power generation could increase fuel consumption and the deficit in a resultant corporate balance sheet. Some improvement is necessary from a practical viewpoint. Therefore, what is needed is the creation of an economical and rational power generation system that involves the least increase in construction cost and which also consumes less fuel, namely, a new power generation system that can avoid the corrosion problem.
The mechanism of corrosion is complicated and various factors are involved in the reaction. However, it can at least be said that the key factor in corrosion is not the HCl concentration in the gas, but whether or not NaCl (m.p. 800.degree. C.) and KCl (m.p. 776.degree. C.) are in such an environment that they take the form of a fume (molten mist). These salts are fused to deposit on heat transfer pipes and thereby accelerate corrosion. The molten salts will eventually become complex salts which solidify at temperatures as low as 550-650.degree. C. and their solidification temperatures vary with the composition (or location) of municipal wastes which, in turn, would be influenced by the quantity and composition of the salts.
These are major causes of the difficulties involved in the commercial implementation of advanced or high-efficiency power generation systems using MW.
Table 2 lists representative causes of corrosion and measures for avoiding corrosion.
TABLE 2 ______________________________________ Causes of Corrosion Corrosion-Preventing Method ______________________________________ 1. Acceleration of corrosion Use of medium-temperature exhaust due to high-temperature gas region exhaust gases 2. Chlorine-induced corrosion Creating an environment with low FeO + 2HCl .fwdarw. FeCl.sub.2 + levels of HCl, Cl.sub.2 and installing the H.sub.2 O superheating pipes in such low- Fe.sub.3 C .fwdarw. 3Fe + C chlorine zones Fe + Cl.sub.2 .fwdarw. FeCl.sub.2 3. CO-induced corrosion Creating an environment with low CO reacts with protective CO levels (that is, creating an layers on the heat transfer oxidizing atmosphere) and installing surfaces with reduction of the steam superheater in these low- ferric oxide (making up such CO zones. layers). 4. Alkali-containing accretion 1. Do not permit adhesion of depositing on the pipe walls deposits by wiping the pipe Acceleration of corrosion surface with a flow of due to deposits of alkali fluidizing medium (maintain metal salts such as sodium a weakly fluidized bed). and potassium salts. 2. Utilize the heat of the fluidizing medium which has a temperature at which the alkali salts do not melt. 3. Remove dust particles in the exhaust gas having a temperature at which the alkali salts are solidified and remove the chlorine salts (chlorides) and then use the cleaned exhaust gas. ______________________________________
The utilization of a medium-temperature region of exhaust gases per Table 2, is known to a certain degree. However, a superheated steam temperature of only 400.degree. C. can be recovered from an exhaust gas temperature of about 600.degree. C. at which the salts will solidify. Hence, the method based on heat recovery from exhaust gases would not be commercially applicable to high-efficiency thermal recycling systems unless the problems of corrosion of molten salts is effectively solved.
The methods of avoidance of corrosion which are listed in Table 2 under items 2), 3) and 4-1) and 4-2) are considered to be effective if they are implemented by using an internally circulating fluidized-bed boiler system in which a combustion chamber is separated from a heat recovery chamber by a partition wall.
The internally circulating fluidized-bed boiler system is attractive since "the fluidized beds can be controlled below temperatures at which alkali salts will melt". However, this method is incapable of avoiding the resynthesis of dioxins.
As is well known, dioxins are resynthesized in heat recovery sections. Studies on methods of treating shredder dust and its effective use have established a relationship between residual oxygen concentration and HCl concentration in exhaust gases in fluidized-bed combustion at 800.degree. C. According to reported data, HCl concentration was about 8,000 ppm (almost equivalent to the theoretical) when the residual oxygen concentration was zero, but with increasing residual oxygen concentration HCl concentration decreased sharply until it was less than 1,000 ppm at 11% O.sub.2 (at typical conditions of combustion).
"Shredder dust" is a general term for rejects of air classification that is performed to recover valuables from shredded scrap automobiles and the like; shredder dust is thus a mixture of plastics, rubber, glass, textile scrap, etc.
The present inventors conducted a combustion test on shredder dust using a test apparatus of 30 t/d (tons/day) and found that the concentration of HCl was comparable to 1,000 ppm (i.e., similar to the above mentioned study). To investigate the materials balance of the chlorine content, the inventors also analyzed the ash in the bag filter and found that it contained as much as 10.6% chlorine, with Cu taking the form of CuCl.sub.2.
With regard to CuCl.sub.2, it has been reported that this compound is related to the generation of PCDD/PCDF in the incineration processes and serves as a catalyst for dioxin resynthesis which is several hundred times as potent as other metal chlorides (ISWA 1988 Proceedings of the 5th Int. Solid Wastes Conference, Andersen, L., Moller, J (eds.), Vol. 1, p. 331, Academic Press, London, 1988). Two of the data in such report are cited here and reproduced in FIG. 5, which shows the effect of Cu concentration on the generation of PCDD (o) and PCDF (.DELTA.), and in FIG. 6, which shows the generation of PCDD (o) and PCDF (.DELTA.) in fly ash as a function of carbon content. The report shows that CuCl.sub.2 and unburnt carbon are significant influences on the resynthesis of dioxins.
It should be noted that carbon tends to remain unburnt in the incineration process since combustion temperatures cannot be higher than 1,000.degree. C.